This invention relates to method and apparatus for producing reinforced polymeric composites, as well as the product produced thereby. More specifically, this invention relates to a pultrusion process, commonly used for producing continuous and constant cross-section products, as well as for producing non-constant cross-section products. The invention is applicable to pultrusion of thermoset composites, as well as pultrusion of thermoplastic composites.
Typically, in a pultrusion process to manufacture polymeric composite products, the fibers (unidirectional, unidirectional with in-situ weaving, or woven mats) are drawn through a dip bath, where they become impregnated with the resin and then through a heated die which controls the shape and size of the product. As the material moves through the die, its outside surface is heated until it is effectively at the same temperature as the die. The core temperature, near the center of the product, will lag behind the die temperature by an amount that depends upon the part geometry and upon the effective thermal properties (thermal diffusivity, thermal conductivity, volumetric specific heat) of the resin/fiber mixture. As the temperature of the product rises, the rate of reaction of the exothermic curing process will increase rapidly, with attendant generation of heat. This is the characteristic of thermoset resins.
If the part is drawn through the die too rapidly, or if the die temperature is too low, the part will not cure completely. If the part thickness is large, it takes more time for the heat to conduct to the center of the part, which means the line speed must be reduced for proper curing of the product. In addition, for large thickness products, the heat generated due to exothermic reaction at the product center may not have time to conduct out, which may result in excessive temperatures inside the composite. These excessive temperatures can crack and delaminate the composite, thus damaging the composite part. Currently, for moderate thicknesses, pre-heating of the resin/fiber mass by RF heating is used. Where pultrusion of moderate thickness composites is possible, only conservative line speeds with limited productivity are used. Applications such as structural composites required in infrastructure demand much higher thicknesses, for which the current technology is not adequate.
An object of the invention is to provide a simple and modified pultrusion process and the corresponding apparatus for producing thicker cross-section polymeric composites which overcomes the limitations of the prior art as described above.
Another object of the invention is to increase the line speeds and hence the productivity.
Another object of the invention is to provide a process and apparatus for producing hybrid composites that can be tailored to different applications.
These and other objects have been achieved according to the present invention by providing a method of producing a pultruded product, comprising: pulling a first fiber coated with a first resin through a first heated die having a first die opening, a first product emerging from the first heated die; and subsequently pulling the first product and a second fiber coated with a second resin through a second heated die having a second die opening which is larger than the first die opening.
These and other objects have also been achieved according to the present invention by providing an apparatus for producing a pultruded product, comprising: a first heated die having a first die opening; a second heated die having a second die opening which is larger than the first die opening; a supply system which supplies a resin coated fiber to each of the heated dies; and a puller which pulls the resin coated fiber and a product emerging from each of said heated dies through subsequent of said heated dies.
These and other objects have also been achieved according to the present invention by providing a composite pultruded product, comprising: an first layer formed by pulling a first fiber coated with a first resin through a first heated die having a first die opening; a second layer formed by pulling the first layer and a second fiber coated with a second resin through a second heated die having a second die opening which is larger than the first die opening.
As mentioned above, this invention provides modification to the current pultrusion process. Also, as mentioned above, the limitation of the current process is the thickness of the product. There is a maximum thickness, depending upon the properties of the composite, above which it is neither practical nor feasible to produce composites with the current art. The present invention overcomes this limitation. Instead of pultruding the thick product in one stage, multiple stages are used to produce the product of final size with a practical thickness product cured in each stage. This allows the product to be pulled at the line speed that is practical for the optimum product thickness in each stage. This line speed will be much higher than that is possible in single-stage for producing the thick part. The process of the invention is still continuous. The apparatus governing the invention will include multiple heated die cavities with successively increasing cavity size. The apparatus can be an integrated pultrusion machine with the dies mounted on the same bed or multiple pultrusion machines assembled in series to produce the product of final dimensions.
The apparatus and the method can be effectively used for producing the hybrid composites, i.e., different fibers and different resins can be used for the same structure depending upon the application.
These and other objects, features and advantages of the present invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings.